The invention relates to a gas turbine set with a cooling air system through which at least one cooling air mass flow flows from a compressor to thermally highly loaded components of the gas turbine set.
In parallel with the requirements on the performance and efficiency factor of gas turbine sets, there are increased requirements on the cooling of the machine components subject to high thermal loads on the one hand, and on the design of the cooling system on the other hand. Thus sufficient cooling has to be ensured in the interests of operating safety. On the other hand, the cooling air consumption has to be limited as far as possible. It was proposed in EP 62932 to cool the components of a gas turbine with steam in a closed circuit. This requires a comparatively expensive sealing of the components conducting the cooling steam. A purely convective cooling of the components takes place at the same time; the effect of a cooling film for reducing heat entry is hereby dispensed with. In a number of further documents, such as EP 684 369 or EP 995 891 and U.S. Pat. No. 6,161,385 corresponding thereto, it is proposed to use steam or a steam-air mixture for the cooling of film-cooled components. However, such methods consume comparatively large amounts of steam, which has to fulfill high requirements on purity and superheating so that blockage of the film cooling bores, often only a few tenths of a millimeter wide, does not occur. Even if the required steam quantity and quality are available, cooling of the gas turbine set with steam, instead of with compressor bleed air, is not inherently more reliable.
Consequently, cooling with compressor bleed air has as usual a series of well-founded advantages, the amount of cooling air withdrawn being minimized in the interest of the working process. Consequently, the cooling air system is designed always closer to the limits, in order to ensure sufficient cooling in thexe2x80x94from the cooling technology viewpointxe2x80x94unfavorable operating point, while not using more cooling air than absolutely necessary. This means, on the one hand, a high sensitivity to deviations of the working process from the design point of the cooling, if, for example, the amounts of cooling air vary due to displacements of the pressure ratios in a machine. On the other hand, an overcooling of the thermally stressed components results in a series of other operating points, so that the performance and efficiency factor potentials remain unexploited.
It was therefore occasionally proposed, for example, in EP 1 028 230, to arrange variable throttle points in the cooling air path. DE 199 07 907 proposes direct adjustment of the initial pressure of the cooling air by means of adjustable compressor blade rows which are arranged immediately neighboring a bleed point for cooling air. Although the implementation of this measure is promising, it is, of course, very expensive, and scarcely suited just for a retrofitting of existing gas turbine sets. Besides, the building of movable parts into the cooling air system holds the latent danger of blockage of the cooling air ducts on failure of mechanical components.
A further relevant question is the feed of cooling air to structures in the region of the combustor or to the front side of the first guide blade row of a turbine. While it is sought to minimize the pressure loss of the working medium, and thus to keep the pressure at the turbine inlet as close as possible to the compressor end pressure, a sufficient cooling air mass flow has to pass through narrow cooling air channels and cooling bores. This of course requires a corresponding pressure drop over the cooling air system, so that the initial pressure of the cooling air system also cannot be higher than the compressor end pressure. Thus also in this regard only an appropriate, but not finally completely satisfactory, compromise can be found between the performance and efficiency factor data of a gas turbine set, on the one hand, and ensuring that there is sufficient cooling.
The present invention has as its object to avoid the disadvantages of the state of the art, in a gas turbine set of the kind named at the beginning.
This is attained by the whole of the features of claim 1.
The core of the invention is thus to provide, in an air-cooled gas turbine set, means for increasing the total pressure in the channels which conduct cooling air, and thus to vary the cooling air mass flow at a given cooling air bleed pressure and counter-pressure.
In a preferred embodiment of the invention, ejectors operable with a working fluid are arranged in the cooling air channels.
In this manner, the unavoidable cooling air pressure losses in cooling the combustor and the first rows of turbine blades can be compensated, at least to an important degree. A further concept on which the invention is based is to increase the total initial pressure of the cooling air in strongly throttled cooling air ducts, the film cooling bores also representing throttle points in the proper sense. A further option of embodying the invention is to dispense with an internal throttling and mass flow adjustment of the cooling air, as is often implemented in the form of diaphragms built into the cooling air ducts, and instead to carry out the cooling air bleed from the compressor at low pressure, and to set the necessary total initial pressure by a pressure increase in the cooling air channels. Furthermore, the invention is particularly suitable in order to vary the cooling air mass flow during operation, preferably in dependence on suitable process parameters. For example, this is possible comparatively easily in the embodiment in which the ejectors are used as means to increase the pressure, in that the intervention is only on the small working fluid mass flow instead of the total cooling air mass flow.
Besides the advantage of not having to adjust the whole cooling air mass flow, and thus also not having to intervene directly in the main cooling system, an existing gas turbine set is comparatively easy to retrofit to the state according to the invention, in comparison with, for example, variable throttle points in the main cooling air system.
As working fluid for the ejector or the ejectors, a working air mass flow of higher pressure than that of the driven cooling air mass flow is particularly suitable, for example. This can be derived from an externally arranged compressor, but can also particularly appropriately be an air mass flow branched off from a higher pressure compressor stage.
Furthermore, a steam mass flow is also very suitable as the working fluid, preferably being correspondingly superheated. Condensation in the cooling air duct is thus avoidable in all circumstances. In contrast to the supply of steam as the working medium, either for purely steam cooling or for steam-air hybrid cooling, it is an advantage that the qualitatively high-value and highly pure steam is required in only comparatively small amounts. The embodiment is advantageous if the gas turbine set is provided with a waste heat steam generator for operation in a combined plant or for the recovery of process steam. Furthermore, there can appropriately also be used as the working fluid, steam which was produced in a cooling air cooler, as in DE 100 41 413, or in a compressor intermediate cooler, as in EP 516 995.
The cooling system, in an appropriate design, is furthermore still inherently safer, since a minimum of cooling air flow is still ensured, even when there is a complete failure of the working fluid supply, in particular of a steam supply. In all, it remains to be established in this connection that the mass flow of the working fluid is in general less than 20%, preferably less than 10%, in particular even less than 5% of the driven cooling air mass flow, so that the working fluid has per se no significant effect as a coolant.
As already mentioned, means are arranged in a preferred embodiment for adjusting the working medium flow in its feed line to an ejector. The invention is particularly suitable in connection with a gas turbine set which is provided with cooling systems of different pressure stages, for example, a high pressure cooling system and a low pressure cooling system, the high pressure cooling system being supplied from a compressor end stage and the low pressure system being supplied from an intermediate compressor stage. In such a case, a preferred embodiment is to arrange in the low pressure system, ejectors which are driven with a working fluid branched off from the high pressure cooling system. This is found to be particularly advantageous if the pressure buildup in the compressor is displaced, which is particularly the case when a cooling takes place during the compression, for example, in the case of a cooling system in the compressor.
The embodiment of a gas turbine set according to the invention is particularly advantageous if a gas turbine set with sequential combustion is concerned, a first combustor and a first turbine being cooled by a high pressure cooling system, and a second combustor and a second turbine being cooled by a cooling system of a lower pressure stage.